When a computer or the like handles an image, image data on the input image acquired from an image input device such as a scanner or a digital camera are expressed with picture elements in a dot matrix arrangement, and the picture elements express different tones (tone levels). In such a case, an image is often displayed on the CRT screen of the computer in 640 and 480 dots of picture elements in the horizontal and vertical directions, respectively.
Further, as each picture element has data expressing a color or brightness in tone, image processing is accomplished by varying these data.
This image processing includes, for document images or the like, processing to remove the background. The background of a document image in this context means the set of picture elements representing the blank area of the paper of the document, and it may include what is written on the back face of the paper seen through. Removing the background means maximizing the tone levels of picture elements representing the blank area of the paper. Thus, when the document image is to be printed after the image processing, the picture elements whose tone levels are maximized as stated above are not printed, and the area of these picture elements remains as blank paper. Therefore, the picture elements represented in the blank paper area are apparently removed, making it possible to obtain a print having clear contrast between the background and the characters. In such a case, the tone levels of picture elements having tones no less brighter than a prescribed threshold are maximized irrespective of the actual presence or absence of a background.
An example of application of this technique is a case in which it is used to prevent, where a sheet of document paper printed on both sides is to be read by a scanner, the image data on the wrong side of this document image from overlapping the image data on the right side. Since the image on the wrong side seen through would appear in the area of the right side where nothing is supposed to be printed, it is removed by maximizing the tone levels of picture elements in this area.
Thus, an area identified with a certain class of tone levels is determined to be the background, and picture elements in this area are subjected to processing for background removal and their tone levels are maximized.